The Digestive System
1. Alimentary Canal
digests food
absorbs through the blood
mouth, pharynx, esophagus, stomach, small intestine, and large intestine.
digestion is a catabolic process
visceral peritoneum covers the external surfaces of most digestive organs and is continuous with the parietal peritoneum that lines the body wall
Between the two peritoneums is the peritoneal cavity, a slitlike potential space containing a slippery fluid secreted by the serous membranes.
A mesentary is a double layer of peritoneum—a sheet of two serous membranes fused back to back—that extends to the digestive organs from the body wall.
They provide routes for blood vessels, lymphatics, and nerves to reach the digestive viscera.
They hold organs in place.
They store fat
Retroperitoneal Organs:
Most of the pancreas
Most of the duodenum
Parts of the large intestine (for example, much of the colon and rectum)
4 Tunics
mucosa, submucosa, muscularis externa, and serosa
Mucosa
Secrete mucus, digestive enzymes, and hormones, Absorb the end products of digestion into the blood, Protect against infectious disease
Epithelium- usually simple columnar, rich in mucus‑secreting cells (like goblet cells) that lubricate and protect the lining and may also secrete enzymes and hormones.
Lamina propria- loose areolar connective tissue containing capillaries (for nutrient absorption) and lymphoid follicles (MALT) that defend against pathogens.
Muscularis Mucosae- thin smooth muscle layer that creates small local movements to enhance secretion and absorption.
Submucosa
connective tissue
blood and lymphatic vessels, lymphoid follicles, and nerve fibers that supply the surrounding tissues of the GI tract wall
Muscularis Externa
Segmentation and peristalsis
circular and longitudinal layer
Serosa
visceral peritoneum
retroperitoneal organs have both serosa and adventitia
Chemoreceptors (mouth)
Detect chemicals in food, especially acidic substances. When activated, they send signals that increase salivation.Mechanoreceptors (mouth)
Respond to touch and pressure from chewing or any mechanical stimulus in the mouth, also triggering salivary reflexes.Osmoreceptors (hypothalamus)
Sense changes in extracellular fluid osmolality (how “concentrated” the body fluids are). A small increase activates them and stimulates the thirst mechanism.Baroreceptors (cardiovascular system)
Detect changes in blood volume/pressure. A drop in volume or pressure helps activate the thirst response.
Control centers
Salivatory nuclei (pons and medulla in the brain stem)
Receive input from oral chemoreceptors and mechanoreceptors and control salivary gland activity via autonomic nerves.Hypothalamic thirst center
Integrates signals from osmoreceptors and baroreceptors to control water intake (thirst).
The Splanchnic Circulation
arteries that branch off the abdominal aorta to serve the digestive organs and hepatic portal circulation
stomach, liver, spleen
the hepatic portal circulation
n, which collects nutrient‑rich venous blood from the digestive organs and delivers it to the liver for processing.
Enteric Nervous System (gut brain)
These plexuses control GI motility (peristalsis, segmentation) and secretory activity.
intrinsic (short reflexes having to do with local controls)
extrinsic (long reflects)
Tongue
Filiform Papillae
roughen the tongue surface
smallest and most numerous
don’t contain taste buds
Fungiform papillae
Mushroom-shaped, scattered over the tongue.
Have a vascular core, giving a reddish color.
Contain taste buds.
Vallate (circumvallate) papillae
8–12 large papillae in a V-shaped row at the back of the tongue.
Surrounded by a furrow.
Contain many taste buds.
Foliate papillae
Pleatlike folds on the lateral aspects of the posterior tongue.
Contain taste buds, especially in childhood.
The Salivary Glands
Parotid glands
Largest, triangular glands.
Located anterior to the ear, between the masseter muscle and the skin.
Parotid duct runs across the cheek, pierces the buccinator, and opens into the vestibule opposite the second upper molar.
Submandibular glands
About the size of a walnut.
Located along the medial aspect of the mandibular body (inside/below the jaw).
Their duct runs in the floor of the mouth and opens at the base of the lingual frenulum (under the tongue).
Sublingual glands
Small, almond-shaped.
Located under the tongue, anterior to the submandibular glands.
Open via 10–20 small ducts into the floor of the mouth.
Composition of saliva
Saliva is mostly water (about 97–99.5%), so it’s a thin, watery fluid that’s slightly acidic (pH ~6.75–7.00) and usually hypoosmotic.
Its main dissolved components are:
Electrolytes: Na⁺, K⁺, Cl⁻, phosphate (PO₄³⁻), and bicarbonate (HCO₃⁻).
Digestive enzymes:
Salivary amylase (begins starch digestion)
Lingual lipase (minor role in fat digestion)
Proteins:
Mucin (forms thick mucus when hydrated, lubricating the mouth and food)
Lysozyme (enzyme that helps kill bacteria)
IgA (antibody that helps protect against microorganisms)
Metabolic wastes: Urea and uric acid.
Together, these components help lubricate and hydrate food, start digestion, and protect the oral cavity from infection.
intrinsic glands always keep the mouth moist
extrinsic glands produce secretions when chemo/mechanoreceptors are stimulated by food
Gross anatomy
Cardia: small area around the cardial orifice where the esophagus enters.
Fundus: dome-shaped part superior to the cardia, under the diaphragm.
Body: large midportion.
Pyloric part: pyloric antrum narrowing into the pyloric canal and ending at the pylorus.
The pyloric sphincter (valve) at the pylorus regulates emptying into the duodenum.
Internally, the mucosa forms longitudinal folds called rugae when the stomach is empty.
Layers (inside → outside)
Mucosa: surface epithelium (simple columnar, all mucous cells), lamina propria, and muscularis mucosae.
Submucosa: contains the submucosal nerve plexus.
Muscularis externa: three smooth muscle layers—inner oblique, middle circular, outer longitudinal—plus the myenteric plexus.
Serosa: outermost covering.
Mucosa and gastric glands
The surface epithelium secretes a thick, alkaline mucus that forms a protective mucosal barrier. The mucosa is dotted with gastric pits that lead to gastric glands. These glands contain:Mucous neck cells (mucus)
Parietal cells (HCl and intrinsic factor)
Chief cells (pepsinogen)
Enteroendocrine cells (hormones and paracrines, including gastrin)
These secretions together form gastric juice and support digestion while protecting the stomach lining.
astric glands. They secrete a thin, more acidic, soluble mucus (different from the thick protective mucus on the surface). Their exact special function is not fully understood, but they contribute to the gland’s mucus environment.
Parietal cells
Located mainly in the upper (apical) part of the glands, among the chief cells. They secrete hydrochloric acid (HCl) and intrinsic factor HCl makes the stomach very acidic, which:activates pepsin from pepsinogen
denatures proteins
helps break down plant cell walls
kills many bacteria
Intrinsic factor is essential for vitamin B₁₂ absorption in the small intestine.
Chief cells
Found mostly in the basal (deeper) part of the glands. They secrete:Pepsinogen, which is activated by HCl (and by pepsin itself) to form pepsin, a protein-digesting enzyme
Gastric lipases, which contribute to fat digestion
Enteroendocrine cells
Some act as paracrines, affecting nearby cells (for example, histamine and serotonin).
Others act as hormones, entering the bloodstream to influence distant digestive organs (for example, somatostatin and gastrin).
Gastrin is especially important: it helps regulate stomach secretion and motility.
More broadly in the GI tract, enteroendocrine cells secrete several peptide hormones that regulate many digestive functions (secretion, motility, etc.) and also release amines like serotonin acting locally as paracrines.
Because they resemble neurons and use transmitter‑like chemicals, they’re sometimes called paraneurons.
Key protein- and fat-digesting enzymes
Pepsin/Pepsinogen (stomach)
Chief cells secrete pepsinogen (inactive). In the acidic stomach lumen, HCl converts pepsinogen to pepsin. Pepsin begins protein digestion, breaking proteins into polypeptides and some amino acids, and works best at pH 1.5–2.5.Gastric and lingual lipases (stomach)
Chief cells also secrete lipases. Gastric and lingual lipases act on fats (lipids) in the acidic stomach and together account for about 15% of total GI fat digestion.Pancreatic proteases (small intestine)
In the small intestine, inactive pancreatic enzymes (trypsinogen, chymotrypsinogen, procarboxypeptidase) are activated in sequence to trypsin, chymotrypsin, and carboxypeptidase, which further break proteins into smaller peptides and amino acids.
Phases of gastric secretion
There are three phases, named for where the main stimuli come from:
Cephalic phase (head)
Happens before food enters the stomach.
Sight, smell, taste, or thought of food activate the brain and vagus nerves.
Vagus stimulation increases gastric gland secretion, “prepping” the stomach.
Gastric phase (stomach)
Begins when food reaches the stomach and lasts 3–4 hours.
Distension activates stretch receptors; peptides, caffeine, and rising pH stimulate G cells to release gastrin.
Gastrin (plus ACh and histamine) strongly increases HCl and enzyme secretion.
Very low pH and stress inhibit this phase.
Intestinal phase (small intestine)
Briefly, partially digested food in the duodenum releases intestinal gastrin, giving a small stimulatory boost.
Then, fatty/acidic/hypertonic chyme and duodenal distension trigger the enterogastric reflex and hormones (secretin, CCK), which inhibit gastric secretion.
breaks on gastric activity to protect the small intestine from acidity
matches the amount of chyme to the digestive abilities of the small intestine
Function of the liver (digestive role)
The liver’s main digestive function is to produce bile and export it to the duodenum.
It also processes nutrient-rich blood:
Stores glucose as glycogen
Uses amino acids to make plasma proteins
Stores fat‑soluble vitamins
Detoxifies wastes and drugs and converts ammonia to urea.
Function of bile
Bile is a fat emulsifier: it breaks large fat droplets into tiny ones, making them easier for enzymes to digest and easier to absorb.
Bile salts are recycled through enterohepatic circulation, reducing the need to make new salts.
What bile is made of
Yellow‑green, alkaline fluid containing:
Bile salts (cholesterol derivatives; the key digestive component)
Phospholipids (e.g., lecithin)
Bile pigments
bilirubin
Cholesterol and triglycerides
Various electrolytes
Pancreas overview
The pancreas is a soft, tadpole‑shaped gland lying deep in the abdomen, mostly behind the stomach. Its “head” sits in the C‑shaped duodenum and the “tail” reaches toward the spleen.
Exocrine role (digestion)
Most of the pancreas is exocrine.
Acinar cells form clusters called acini that produce enzyme‑rich pancreatic juice, which breaks down all categories of food in the small intestine.
A system of ducts carries this juice and adds water and bicarbonate, making the secretion alkaline (about pH 8) to neutralize acidic chyme.
Endocrine role (blood glucose control)
Scattered “islands” of endocrine tissue called pancreatic islets (islets of Langerhans) secrete hormones into the blood.
Alpha (α) cells secrete glucagon, which raises blood glucose.
Beta (β) cells secrete insulin, which lowers blood glucose. These hormones act as opposing (antagonistic) regulators.
Gross anatomy of the small intestine
The small intestine is a long, convoluted tube extending from the pyloric sphincter of the stomach to the ileocecal valve where it joins the large intestine. It is the longest part of the alimentary canal (about 2–4 m in life) but only about half the diameter of the large intestine (2.5–4 cm).
It has three main subdivisions, in order:
Duodenum – about 25 cm long, mostly retroperitoneal, curving around the head of the pancreas. It contains important structures such as the major duodenal papilla.
Jejunum – about 2.5 m long, intraperitoneal, hanging in coils by a fan-shaped mesentery.
Ileum – about 3.6 m long, also intraperitoneal, ending at the ileocecal valve where it meets the large intestine.
Microscopic/structural modifications
The small intestine is specialized to maximize absorption by greatly increasing surface area:
Circular folds: Deep, permanent folds of mucosa and submucosa. They force chyme to spiral along the lumen, slowing its movement and giving more time for absorption.
Villi: Fingerlike projections of the mucosa (over 1 mm high) that give the lining a velvety, towel‑like texture. Each villus contains:
A core of blood capillaries and a central lymphatic vessel (lacteal)
Surface cells (enterocytes) that absorb nutrients into blood and lacteals
Goblet cells and some enteroendocrine cells in the epithelium
Microvilli: Microscopic extensions of the enterocytes’ plasma membrane forming the “brush border.” They:
Further increase surface area
Bear brush border enzymes that complete digestion of carbohydrates and proteins.
Key cell types in villi and crypts
Enterocytes: simple columnar absorptive cells with microvilli (brush border). In villi, they absorb nutrients and electrolytes; in crypts, they secrete intestinal juice.
Goblet cells: secrete mucus to lubricate and protect the lining.
Enteroendocrine cells: release hormones like secretin and CCK that regulate digestion.
Paneth cells: deep in crypts; secrete antimicrobial agents (defensins, lysozyme) to control gut bacteria.